Magnetic recording medium

ABSTRACT

In a magnetic recording medium according to the present invention, if a straight line W having a length of 500 nm and a width of 15 nm is displayed parallel to a width direction of the magnetic layer and a straight line L having a length of 500 nm and a width of 15 nm is displayed parallel to a longitudinal direction of the magnetic layer, the number of magnetic particles that intersect the straight line W is N1, and the number of magnetic particles that intersect the straight line L is N2, then, a relationship of N1/0.5&gt;60 and N2/0.5&gt;60 is established where N1/0.5 is the number of magnetic particles per micrometer obtained by dividing N1 by 0.5 μm and N2/0.5 is the number of magnetic particles per micrometer obtained by dividing N2 by 0.5 μm.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a high capacity magnetic recordingmedium having an excellent electromagnetic conversion property.

2. Description of Related Art

A coating-type magnetic recording medium provided with a magnetic layercontaining magnetic powder and a binder on a nonmagnetic support bodyneeds to have a higher recording density accompanying the transition ofa recording reproduction method from an analog method to a digitalmethod. In particular, a high-density digital video tape, a computerbackup tape, and the like need to satisfy this increasing demand.

A recording wavelength is shortened accompanying such an increase in therecording density, and in order to support this short wavelengthrecording, attempts have been made to micronize magnetic powder year byyear, and currently, ferromagnetic hexagonal ferrite powder having anaverage particle diameter of about 20 nm has been realized, and amagnetic recording medium using this magnetic powder has beenpractically used (for example, JP 2015-91747A).

Moreover, in order to further increase the recording density of themagnetic recording medium using the above-described ferromagnetichexagonal ferrite powder, the ferromagnetic hexagonal ferrite powderneeds to be further micronized. However, the volume of magnetic powderparticles is reduced by further micronizing the ferromagnetic hexagonalferrite powder, and there is a problem that the ferromagnetic hexagonalferrite powder particles tend to be influenced by heat fluctuation.Thus, it is necessary to suppress heat fluctuation using a magneticmaterial having a high magnetic coercive force and a high anisotropyenergy even though the magnetic material is micronized.

In such a circumstance, in recent years, ε-Fe₂O₃ has been studied as anew magnetic material for a magnetic recording medium, and iron oxidenano-magnetic particle powder constituted by a single phase of ε-Fe₂O₃having a ferrite magnetic property and having an average particlediameter of 15 nm or less or preferably 10 nm or less has been proposed(for example, JP 2014-224027A). Also, a magnetic recording medium usingε-Fe₂O₃ as the magnetic powder has been proposed (for example, JP2014-149886A, JP 2015-82329A, WO 2015/198514).

Also, JP 2012-43495A relating to a method for measuring spacing is anexample of prior art documents related to the present invention.

The track density of a magnetic layer has been increased accompanyingsuch a high recording density for increasing the capacity of a magneticrecording medium. However, the track width decreases accompanying anincrease in the track density, and as a result, there is a problem inthat the output property decreases and the electromagnetic conversionproperty also decreases.

The present invention was made in order to solve such a problem, andprovides a magnetic recording medium having an excellent electromagneticconversion property even though the track density is increasedaccompanying an increase in the recording density.

SUMMARY OF THE INVENTION

The magnetic recording medium of the present invention is a magneticrecording medium including a nonmagnetic support body and a magneticlayer containing magnetic particles, in which 0.0013 μT·m<Mr·t<0.0032ρT·m is satisfied where Mr is a residual magnetic flux density in athickness direction of the magnetic layer and t is an average thicknessof the magnetic layer, a squareness ratio in the thickness direction ofthe magnetic layer is 0.65 or more, if a straight line W having a lengthof 500 nm and a width of 15 nm is displayed parallel to a widthdirection of the magnetic layer and a straight line L having a length of500 nm and a width of 15 nm is displayed parallel to a longitudinaldirection of the magnetic layer, on an image obtained by observing asurface of the magnetic layer using a scanning electron microscope at 10k-fold magnification, the straight line W does not intersect a particlehaving a particle diameter of 50 nm or more and a gap having a maximumwidth of 50 nm or more on the image, and on the image, the number ofmagnetic particles that intersect the straight line W is N1, and thestraight line L does not intersect a particle having a particle diameterof 50 nm or more and a gap having a maximum width of 50 nm or more onthe image, and on the image, the number of magnetic particles thatintersect the straight line L is N2, then, a relationship of N1/0.5>60and N2/0.5>60 is established where N1/0.5 is the number of magneticparticles per micrometer obtained by dividing N1 by 0.5 μm and N2/0.5 isthe number of magnetic particles per micrometer obtained by dividing N2by 0.5 μm.

According to a magnetic recording medium of the present invention, eventhough the track width is reduced due to an increase in the trackdensity accompanying an increase in the recording density for increasingthe capacity, it is possible to provide a magnetic recording mediumhaving an excellent electromagnetic conversion property.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram showing one example of amagnetic recording medium.

FIG. 2 is an illustrative diagram showing an image obtained by observinga surface of a magnetic layer using a scanning electron microscope.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of a magnetic recording medium of the present inventionwill be described.

The magnetic recording medium of this embodiment includes a nonmagneticsupport body and a magnetic layer containing magnetic particles. Also,0.0013 μT·m<Mr·t<0.0032 ρT·m is satisfied where Mr is a residualmagnetic flux density in the thickness direction of the above-describedmagnetic layer and t is an average thickness of the magnetic layer, anda squareness ratio in the thickness direction of the magnetic layer is0.65 or more. Furthermore, if a straight line W having a length of 500nm and a width of 15 nm is displayed parallel to the width direction ofthe magnetic layer and a straight line L having a length of 500 nm and awidth of 15 nm is displayed parallel to the longitudinal direction ofthe magnetic layer, on an image obtained by observing a surface of themagnetic layer using a scanning electron microscope at 10 k-foldmagnification, the straight line W does not intersect particles having aparticle diameter of 50 nm or more and gaps having a maximum width of 50nm or more on the image, and on the image, the number of magneticparticles that intersect the straight line W is N1, and the straightline L does not intersect particles having a particle diameter of 50 nmor more and gaps having a maximum width of 50 nm or more, and on theimage, the number of magnetic particles that intersect the straight lineL is N2, then, the relationship of N1/0.5>60 and N2/0.5>60 isestablished where N1/0.5 is the number of magnetic particles permicrometer obtained by dividing N1 by 0.5 μm and N2/0.5 is the number ofmagnetic particles per micrometer obtained by dividing N2 by 0.5 μm.

Here, N1 and N2 each indicate the number of magnetic particlesassociated with magnetic recording in a region in which neitherparticles that have a particle diameter of 50 nm or more and are notassociated with magnetic recording, such as additives, nor gaps thathave a maximum width of 50 nm or more and are not associated withmagnetic recording are present, that is, in a region in which mainlymagnetic particles are present. Also, N1/0.5 and N2/0.5 are used as thenumbers of magnetic particles per micrometer in order to obtain thenumber of magnetic particles per micrometer by respectively dividing N1and N2 by lengths of the straight lines W and L of 500 nm (0.5 μm).

By setting N1/0.5 to be greater than 60 and N2/0.5 to be greater than60, it is possible to ensure a sufficient number of minute magneticparticles and a uniform distribution state of magnetic particles in thewidth direction and the longitudinal direction of the magnetic layer.Accordingly, even though the length of magnetization (track width) inthe width direction of the magnetic layer is set to 1 μm or less, thelength being a length of magnetization of signals recorded in themagnetic layer, a good electromagnetic conversion property (SN property)can be obtained. Also, it is more preferable to set N1/0.5 to be greaterthan 70 and N2/0.5 to be greater than 70.

That is, conventionally, in a storage drive of a magnetic recordingmedium (for example, magnetic tape), the track width has been 10 timeslonger than the recording bit length (½ recording wavelength), and inorder to increase the recording density, it is necessary to reduce thetrack width, and in this case, the track width is 1 μm or less, which isbelow 10 times the recording bit length. If the track width is reducedin this manner, minute magnetic field disturbances become noise, andthus, in order to keep a good SN property, it is necessary to distributea sufficient number of minute magnetic particles uniformly in the widthdirection and the longitudinal direction of the magnetic layer. In thepresent embodiment, by setting N1/0.5 to be greater than 60 and N2/0.5to be greater than 60, it is possible to achieve a state in which asufficient number of minute magnetic particles are uniformly distributedin the width direction and longitudinal direction of the magnetic layer.

Also, by setting a relationship between the residual magnetic fluxdensity Mr in the thickness direction of the magnetic layer and theaverage thickness t of the magnetic layer to be 0.0013 ρT·m<Mr·t<0.0032ρT·m, and a squareness ratio in the thickness direction of the magneticlayer to be 0.65 or more, the resolution of recording magnetizationincreases, and thus even though the track width is set to 1 μm or less,a more preferable electromagnetic conversion property (SN property) canbe obtained. Furthermore, it is more preferable that 0.0020μT·m<Mr·t<0.0030 pT·m is satisfied, and the squareness ratio is morepreferably 0.75 or more.

If a length of magnetization in the width direction of the magneticlayer is set to 1 μm or less, the length being a length of magnetizationof a signal recorded in the magnetic layer, reproduction is preferablyperformed by a tunnel magnetoresistance effect head (TMR head). Eventhough the length of the magnetization is 1 μm or less in order toincrease the track density of the magnetic layer, a high SN ratio can beobtained by reproducing the magnetic recording medium using a highlysensitive TMR head.

The length of the magnetization can be measured as follows, for example.That is, the length of the magnetization in the width direction of themagnetic layer on which signals are recorded is measured using afrequency detection method and “Nano Scope III” (product name)manufactured by Digital Instruments Corporation as the magnetic forcemicroscope. A probe having a cobalt alloy coating (the radius ofcurvature of the tip: 25 to 40 nm, magnetic coercive force: about 400[Oe], magnetic moment: about 1×10⁻¹³ emu) is used as the measurementprobe, the scanning range is 5 μm×5 μm, and the scanning speed is 5μm/sec.

Furthermore, the above-described magnetic particles are preferably madeof ε-iron oxide. If the track width is 1 μm or less, even though theaverage particle diameter of magnetic particles is 20 nm or less, themagnetic coercive force of the magnetic particles does not decrease dueto usage of ε-iron oxide particles as the magnetic particles.

Also, in order to further support short wavelength recording, theaverage particle diameter of the magnetic particles made of ε-iron oxideabove is preferably 15 nm or less. Furthermore, an average particlediameter of the magnetic particles made of ε-iron oxide above is morepreferably 12 nm or less. In general, the lower limit of the averageparticle diameter of the magnetic particles made of ε-iron oxide aboveis about 8 nm. This is because ε-iron oxide having an average particlediameter of less than 8 nm is not easily manufactured.

A magnetic coercive force in the thickness direction of the magneticlayer is preferably 3000 oersteds [Oe] or more. This is because bysetting the magnetic coercive force to 3000 oersteds [Oe] or more, ahigh reproduction output with low self demagnetization loss can beobtained even in a short wavelength recording region at a high recordingdensity.

Also, when spacing on the surface of the magnetic layer is measuredusing a TSA (tape spacing analyzer) after the surface of the magneticlayer is cleaned using n-hexane, a value of the spacing is preferably 5nm or more and 12 nm or less. If the value of the spacing is lower than5 nm, the surface of the magnetic layer tends to become excessivelysmooth, the area of contact between the magnetic head and the magneticlayer tends to increase, a friction coefficient tends to increase, andthe durability of the magnetic layer tends to decrease. On the otherhand, if the value of the spacing exceeds 12 nm, the distance betweenthe magnetic head and the surface of the magnetic layer tends toincrease excessively, and the recording reproduction property tends todecrease. The value of the spacing is more preferably 7 nm or more and12 nm or less, and most preferably 8 nm or more and 11 nm or less.

Although there is no particular limitation on a method for measuring thevalue of the spacing and a method for controlling the same, measurementand controlling can be performed using a method disclosed in JP2012-43495A, for example.

The thickness of the magnetic layer is preferably 30 nm or more and 200nm or less. By setting the thickness of the magnetic layer to 200 nm orless, the short wavelength recording property can be improved, and bysetting the thickness of the magnetic layer to 30 nm or more, a servosignal can be recorded. If ε-iron oxide particles are used as themagnetic particles of the present embodiment, the saturationmagnetization quantity of the ε-iron oxide particles is ½ to ⅓ smallerthan the saturation magnetization quantity of conventional ferromagnetichexagonal ferrite particles, and thus, if a servo signal having a longrecording wavelength is recorded, the thickness of the magnetic layerneeds to be 30 nm or more.

If the servo signal is not recorded in the magnetic layer, the thicknessof the magnetic layer is preferably 10 nm or more and 50 nm or less.Even though the thickness of the magnetic layer is 10 nm or more and 50nm or less, a data signal can be recorded and reproduced using a highlysensitive magnetic head such as a tunnel magnetoresistance effect head(TMR head).

Hereinafter, the magnetic recording medium of the present embodimentwill be described based on the drawings. FIG. 1 is a schematiccross-sectional diagram showing one example of the magnetic recordingmedium of the present embodiment.

In FIG. 1, a magnetic recording medium 10 of the present embodiment is amagnetic tape including a nonmagnetic support body 11, an undercoatlayer 12 formed on one main surface (upper surface herein) of thenonmagnetic support body 11, and a magnetic layer 13 formed on a mainsurface (upper surface herein) of the undercoat layer 12 that isopposite to the nonmagnetic support body 11 side. Also, the main surface(lower surface) of the nonmagnetic support body 11 on which theundercoat layer 12 is not formed is provided with a back coat layer 14.

Magnetic Layer

The magnetic layer 13 contains magnetic particles and a binder. ε-ironoxide particles are preferable as the magnetic particles.

It is preferable that the above-described ε-iron oxide particles areformed by a single phase expressed by a general composition formulaε-Fe₂O₃. This is because if α-iron oxide or γ-iron oxide is mixed in,the magnetic coercive force of the magnetic layer decreases. However,α-iron oxide or γ-iron oxide may be included as impurities as long asthe magnetic coercive force of the magnetic layer does not decrease.

Also, in the present embodiment, ε-iron oxide and other types of ironoxide such as γ-iron oxide and α-iron oxide can be identified byanalyzing their crystal structures through X-ray diffraction.

The magnetic coercive force of the ε-iron oxide particles is preferably3000 oersteds [Oe] or more. Accordingly, the magnetic coercive force inthe thickness direction of the magnetic layer can be set to 3000oersteds [Oe] or more. Also, if particles made of ε-iron oxide expressedby a general composition formula ε-Fe₂O₃ contain impurities, themagnetic coercive force of ε-iron oxide particles decreases, and thus itis preferable that the ε-iron oxide particles do not contain impurities.However, the magnetic coercive force of the above-described ε-iron oxideparticles can be controlled by substituting a part of Fe sites of thecrystals with trivalent metal elements such as aluminum (Al), gallium(Ga), rhodium (Rh), and indium (In). Thus, the ε-iron oxide particlesmay contain metal elements other than iron as impurities as long as themagnetic coercive force can be kept at 3000 oersteds [Oe] or more.

As described above, the average particle diameter of the ε-iron oxideparticles included in the magnetic layer is preferably set to 8 nm ormore and 15 nm or less. If the average particle diameter of the ε-ironoxide particles exceeds 15 nm, in particular, noise of the magneticrecording medium increases in short wavelength recording, and thus ahigh electromagnetic conversion property tends not to be obtained.

In the present embodiment, the average particle diameter of magneticparticles included in the magnetic layer was determined as follows using100 magnetic particles in one field of view using a photograph obtainedby imaging the surface of the magnetic layer with an accelerationvoltage of 2 kV, 10000-fold (10 k-fold) magnification, and anobservation condition of U-LA100, using a scanning electron microscope(SEM) “S-4800” manufactured by HITACHI Ltd.

If the particles have a needle shape, the average particle diameter isdetermined by calculating an average long axial diameter of 100particles, if the particles have a plate shape, the average particlediameter is determined by calculating an average maximum plate diameterof 100 particles, and if the particles are spherical or an ellipsoidalshape in which a ratio of the longer axis length to the shorter axislength is 1 to 3.5, the average particle diameter is determined bycalculating an average maximum diameter of 100 particles.

A conventionally known thermoplastic resin, thermosetting resin, or thelike can be used as the binder included in the magnetic layer 13.Specifically, examples of the above-described thermoplastic resininclude a vinyl chloride resin, a vinyl chloride-vinyl acetate copolymerresin, a vinyl chloride-vinyl alcohol copolymer resin, a vinylchloride-vinyl acetate-vinyl alcohol copolymer resin, a vinylchloride-vinyl acetate-maleic anhydride copolymer resin, a vinylchloride-hydroxyl group-containing alkyl acrylate copolymer resin, and apolyester polyurethane resin. Also, specific examples of theabove-described thermosetting resin include a phenolic resin, an epoxyresin, a polyurethane resin, a urea resin, a melamine resin, and analkyd resin.

The content of the binder in the magnetic layer 13 is preferably 7 to 50parts by mass and more preferably 10 to 35 parts by mass with respect to100 parts by mass of magnetic particles.

Also, it is preferable to use a thermosetting crosslinking agent thatbinds to functional groups or the like included in the binder and formsa crosslinking structure in combination with the binder. Specificexamples of the above-described crosslinking agent include isocyanatecompounds such as tolylene diisocyanate, hexamethylene diisocyanate, andisophorone diisocyanate; reaction products of an isocyanate compound anda compound having a plurality of hydroxyl groups such astrimethylolpropane; and various polyisocyanates such as productsobtained by condensation of isocyanate compounds. The content of theabove-described crosslinking agent is preferably 10 to 50 parts by masswith respect to 100 parts by mass of the binder.

If the magnetic layer 13 contains the above-described magnetic particlesand binder, the magnetic layer 13 may further contain an additive suchas a polishing agent, a lubricant, or a dispersing agent. In particular,from the viewpoint of durability, a polishing agent and a lubricant arepreferably used.

Specific examples of the above-described polishing agent includeα-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide,α-iron oxide, corundum, artificial diamond, silicon nitride, siliconcarbide, titanium carbide, titanium oxide, silicon dioxide, and boronnitride, and among these, a polishing agent having a Mohs' hardness of 6or more is more preferable. These may be used alone or in combination.The average particle diameter of the above-described polishing agent ispreferably 10 to 200 nm although it depends on the type of polishingagent that is to be used. The content of the above-described polishingagent is preferably 5 to 20 parts by mass and more preferably 8 to 18parts by mass with respect to 100 parts by mass of magnetic particles.

Examples of the above-described lubricant include fatty acids, fattyacid esters, and fatty acid amides. Although the above-described fattyacid may be any of linear, branched, and cis/trans isomers, a linearfatty acid having excellent lubricant performance is preferable.Specific examples of such fatty acids include lauric acid, myristicacid, stearic acid, palmitic acid, behenic acid, oleic acid, andlinoleic acid. Specific examples of the above-described fatty acid esterinclude N-butyl oleate, hexyl oleate, n-octyl oleate, 2-ethylhexyloleate, oleyl oleate, n-butyl laurate, heptyl laurate, n-butylmyristate, n-butoxyethyl oleate, trimethylolpropane trioleate, n-butylstearate, s-butyl stearate, isoamyl stearate, and butyl cellosolvestearate. Specific examples of the above-described fatty acid amideinclude palmitic acid amide and stearic acid amide. These lubricants maybe used alone or in combination.

Among these, it is preferable to use a fatty acid ester and a fatty acidamide in combination. In particular, it is preferable to use 0.2 to 3parts by mass of a fatty acid ester and 0.5 to 5 parts by mass of afatty acid amide with respect to 100 parts by mass of the total solidcontent such as magnetic particles and polishing agents in the magneticlayer 13. This is because if the content of the above-described fattyacid ester is less than 0.2 parts by mass, the fatty acid ester has asmall friction coefficient reduction effect, and if the content thereofexceeds 3.0 parts by mass, there is a concern about the occurrence ofside effects such as the magnetic layer 13 attaching to the head. Also,this is because if the content of the above-described fatty acid amideis less than 0.5 parts by mass, the fatty acid amide has little effectof preventing seizing caused by mutual contact between the magnetic headand the magnetic layer 13, and if the content thereof exceeds 5 parts bymass, there is a concern that the fatty acid amide will undergobleedout.

Also, the magnetic layer 13 may contain carbon black for the purpose ofincreasing the conductivity and surface lubricity. Specific examples ofsuch carbon black include acetylene black, furnace black, and thermalblack. The average particle diameter of carbon black is preferably 0.01to 0.1 μm. If the above-described average particle diameter is 0.01 μmor more, it is possible to form the magnetic layer 13 in which carbonblack is well dispersed. On the other hand, if the above-describedaverage particle diameter is 0.1 μm or less, it is possible to form themagnetic layer 13 having excellent surface smoothness. Also, two or moretypes of carbon black having different average particle diameters may beused as needed. The content of the above-described carbon black ispreferably 0.2 to 5 parts by mass and more preferably 0.5 to 4 parts bymass with respect to 100 parts by mass of magnetic particles.

It is preferable that a center-line average surface roughness Ra of themagnetic layer 13 is less than 2.0 nm, the center-line average surfaceroughness being defined in Japanese Industrial Standard (JIS) B0601. Themore the surface smoothness of the magnetic layer 13 increases, thehigher the output can be obtained, but if the surface of the magneticlayer 13 is excessively smoothened, the friction coefficient becomeshigh and the travel stability decreases. Thus, Ra is preferably 1.0 nmor more.

Next, the surface state of the magnetic layer 13 will be described. FIG.2 is a schematic diagram showing an image obtained by observing thesurface of the magnetic layer 13 in FIG. 1 using a scanning electronmicroscope.

In FIG. 2, a straight line W having a length of 500 nm and a width of 15nm is displayed parallel to the width direction of the magnetic layer 13and a straight line L having a length of 500 nm and a width of 15 nm isdisplayed parallel to the longitudinal direction of the magnetic layer13, on an image 20 obtained by observing the surface of the magneticlayer 13 in FIG. 1 using the scanning electron microscope at 10 k-foldmagnification. Here, if the straight line W does not intersect particles21 having a particle diameter of 50 nm or more and gaps 22 having amaximum width of 50 nm or more on the image 20, and on the image 20, thenumber of magnetic particles 23 intersecting the straight line W is N1,and the straight line L does not intersect particles 21 having aparticle diameter of 50 nm or more and gaps 22 having a maximum width of50 nm or more on the image 20, and on the image 20, the number ofmagnetic particles 23 intersecting the straight line L is N2, then, arelationship in which N1/0.5 is greater than 60 (N1/0.5>60) and N2/0.5is greater than 60 (N2/0.5>60) is established where N1/0.5 is the numberof magnetic particles per micrometer obtained by dividing N1 by 0.5 μmand N2/0.5 is the number of magnetic particles per micrometer obtainedby dividing N2 by 0.5. Accordingly, it is possible to ensure, in thewidth direction and the longitudinal direction of the magnetic layer 13,a sufficient number of minute magnetic particles, obtain a state inwhich magnetic particles are uniformly distributed, and a goodelectromagnetic conversion property (SN property) even if the trackwidth is 1 μm or less. Here, intersecting means that all or part of themagnetic particles are included in the straight line W or the straightline L.

A specific method for establishing the relationship of N1/0.5>60 andN2/0.5>60 in the magnetic layer 13 will be described in detail in thedescription of a method for manufacturing the magnetic recording mediumof the present embodiment, which will be described later.

Lubricant Layer

Although not shown in FIG. 1, in order to reduce the frictioncoefficient of the magnetic layer 13 and further increase the durabilityof the magnetic layer 13, it is preferable to provide, on the magneticlayer 13, a lubricant layer containing a fluorine-based lubricant or asilicone-based lubricant. Examples of the above-described fluorine-basedlubricant include trichlorofluoroethylene, perfluoropolyether,perfluoroalkyl polyether, and perfluoroalkyl carboxylic acid. Examplesof the above-described silicone-based lubricant include silicone oil andmodified silicone oil. These lubricants may be used alone or incombination. More specifically, for example, “NOVEC7100” or “NOVEC1720”(product name) manufactured by 3M Company can be used as thefluorine-based lubricant, and “KF-96L”, “KF-96A”, “KF-96”, “KF-96H”“KF-99”, “KF-50”, “KF-54”, “KF-965”, “KF-968”, “HIVAC F-4”, “HIVAC F-5”,“KF-56A”, “KF995”, “KF-69”, “KF-410”, “KF-412”, “KF-414”, and “FL”(product name) manufactured by Shin-Etsu Chemical Co., Ltd., and“BY16-846”, “SF8416”, “SH200”, “SH203”, “SH230”, “SF8419”, “FS1265”,“SH510”, “SH550”, “SH710”, “FZ-2110”, and “FZ-2203” (product name)manufactured by Dow Corning Toray Co., Ltd. can be used as thesilicone-based lubricant.

There is no particular limitation on the thickness of the lubricantlayer, and it is sufficient that the thickness thereof is 3 to 5 nm, forexample. The thickness of the lubricant layer can be measured using amethod in which a TSA disclosed in JP 2012-43495A above is used, basedon a difference in spacing between the magnetic recording medium and atransparent body before and after the lubricant layer is cleaned usingan organic solvent.

The lubricant layer can be formed by top-coating the magnetic layer 13with the lubricant. As described above, the magnetic layer 13 isuniformly filled with minute magnetic particles, and thus the lubricantsincluded in the magnetic layer 13 are unlikely to move to the surface ofthe magnetic layer 13. However, by top-coating in which the lubricantsare applied to the surface of the magnetic layer, a lubricant layer canbe reliably formed on the surface of the magnetic layer 13.

Undercoat Layer

An undercoat layer 12 having a lubricant retention function and anexternal stress (e.g., pressing force of the magnetic head) cushioningfunction is preferably provided under the magnetic layer 13. Also, byproviding the undercoat layer 12, the strength of the magnetic recordingmedium 10 increases, and thus when the magnetic recording medium 10 isformed, calendering can be performed and the fillability of the magneticlayer 13 can be improved. The undercoat layer 12 contains nonmagneticpowder, a binder, and a lubricant.

Examples of the nonmagnetic powder included in the undercoat layer 12include carbon black, titanium oxide, iron oxide, and aluminum oxide,and in general, carbon black is used alone, or carbon black and anothernonmagnetic powder such as titanium oxide, iron oxide, or aluminum oxideare mixed and used. In order to form a smooth undercoat layer 12 byforming a coating film with little thickness unevenness, it ispreferable to use nonmagnetic powder having a sharp particle sizedistribution. From the viewpoint of ensuring the uniformity, surfacesmoothness, rigidity, and conductivity of the undercoat layer 12, anaverage particle diameter of the above-described nonmagnetic powder ispreferably 10 to 1000 nm and more preferably 10 to 500 nm, for example.

The particle shape of the nonmagnetic powder included in the undercoatlayer 12 may be any of a spherical shape, a plate shape, a needle shape,and a spindle shape. An average long axial diameter of a needle-shapedor spindle nonmagnetic powder is preferably 10 to 300 nm and an averageshort axial diameter thereof is preferably 5 to 200 nm. An averageparticle diameter of a spherical nonmagnetic powder is preferably 5 to200 nm and more preferably 5 to 100 nm. An average particle diameter ofa plate-shaped nonmagnetic powder is preferably 10 to 200 nm in terms ofthe maximum plate diameter. Furthermore, in order to form the smoothundercoat layer 12 having little thickness unevenness, nonmagneticpowder having a sharp particle size distribution is preferably used.

A binder and a lubricant that are similar to those used in theabove-described magnetic layer 13 are used as the binder and thelubricant that are included in the undercoat layer 12. The content ofthe above-described binder is preferably 7 to 50 parts by mass and morepreferably 10 to 35 parts by mass with respect to 100 parts by mass ofthe above-described nonmagnetic powder. Also, the content of theabove-described lubricant is preferably 2 to 6 parts by mass and morepreferably 2.5 to 4 parts by mass with respect to 100 parts by mass ofthe above-described nonmagnetic powder.

If the magnetic particles used in the magnetic layer 13 are ε-iron oxideparticles, the saturation magnetization quantity of the ε-iron oxideparticles is ½ to ⅓ smaller than the saturation magnetization quantityof conventional ferromagnetic hexagonal ferrite particles, and thus, ifa servo signal having a long recording wavelength is recorded, theundercoat layer 12 needs to contain magnetic particles. For example,needle-shaped metal iron-based magnetic particles, plate-shapedhexagonal ferrite magnetic particles, particulate iron nitride-basedmagnetic particles, or the like can be used as the above-describedmagnetic particles.

The thickness of the undercoat layer 12 is preferably 0.1 to 3 μm andmore preferably 0.3 to 2 μm. By setting the thickness of the undercoatlayer 12 in this range, a lubricant retention function and an externalstress cushioning function can be maintained without unnecessarilyincreasing the total thickness of the magnetic recording medium 10.

Nonmagnetic Support Body

A nonmagnetic support body for a magnetic recording medium that has beenused conventionally can be used as the nonmagnetic support body 11.Specific examples thereof include films made of polyesters such aspolyethylene terephthalate and polyethylene naphthalate, and films madeof polyolefins, cellulose triacetate, polycarbonate, polyamide,polyimide, polyamide-imide, polysulfone, or aramid.

The thickness of the nonmagnetic support body 11 varies depending on theapplications, and is preferably 1.5 to 11 μm and more preferably 2 to 7μm. If the thickness of the nonmagnetic support body 11 is 1.5 μm ormore, the film formation property increases and a high strength can beobtained. On the other hand, if the thickness of the nonmagnetic supportbody 11 is 11 μm or less, the total thickness does not necessarilyincrease, and the storage capacity per reel of a magnetic tape can beincreased, for example.

A Young's modulus in the longitudinal direction of the nonmagneticsupport body 11 is preferably 5.8 GPa or more and more preferably 7.1GPa or more. If the Young's modulus in the longitudinal direction of thenonmagnetic support body 11 is 5.8 GPa or more, the travel ability canbe increased. Also, in a magnetic recording medium used in a helicalscan system, a ratio (MD/TD) between the Young's modulus in thelongitudinal direction (MD) and the Young's modulus in the widthdirection (TD) is preferably 0.6 to 0.8, more preferably 0.65 to 0.75,and even more preferably 0.7. If the ratio (MD/TD) is in theabove-described range, it is possible to suppress variation (flatness)in the output from the entry side on which a magnetic head enters atrack to the exit side on which the magnetic head exits the track. In amagnetic recording medium used in a linear recording system, a ratio(MD/TD) between the Young's modulus in the longitudinal direction (MD)and the Young's modulus (TD) in the width direction is preferably 0.7 to1.3.

Back Coat Layer

A main surface (lower surface herein) that is opposite to the mainsurface of the nonmagnetic support body 11 that is provided with theundercoat layer 12 is preferably provided with a back coat layer 14 forthe purpose of increasing the travel ability or the like. The thicknessof the back coat layer 14 is preferably 0.2 to 0.8 μm and morepreferably 0.3 to 0.8 μm. If the thickness of the back coat layer 14 isexcessively thin, the back coat layer 14 has an insufficient travelability increasing effect, and if the thickness thereof is excessivelythick, the total thickness of the magnetic recording medium 10increases, and the storage capacity per reel of a magnetic tapedecreases, for example.

The back coat layer 14 preferably contains carbon black such asacetylene black, furnace black, or thermal black, for example. Ingeneral, carbon black with a smaller particle diameter and carbon blackwith a larger particle diameter whose particle diameters are differentfrom each other are used in combination. The reason why these are usedin combination is that the travel ability increasing effect increases.

Also, the back coat layer 14 contains a binder, and a binder that issimilar to that used in the magnetic layer 13 and the undercoat layer 12can be used as the binder. Among these, in order to reduce the frictioncoefficient and increase the travel ability of the magnetic head, it ispreferable to use a cellulose-based resin and a polyurethane resin incombination.

For the purpose of increasing the strength, the back coat layer 14preferably further contains iron oxide, alumina, and the like.

Next, a method for manufacturing a magnetic recording medium of thepresent invention will be described. In the method for manufacturing themagnetic recording medium of the present invention, for example, amagnetic layer formation coating material, an undercoat layer formationcoating material, and a back coat layer formation coating material areproduced by mixing layer formation components and solvents, and amagnetic layer is formed using a sequential multilayer coating method inwhich the undercoat layer is formed by applying the undercoat layerformation coating material to one side of a nonmagnetic support body anddried, and then the magnetic layer formation coating material is appliedto the undercoat layer and dried, and the back coat layer is furtherformed by applying the back coat layer formation coating material to theother side of the nonmagnetic support body and drying the back coatlayer formation coating material. Thereafter, calendering is performedon the entireties of the layers so as to obtain a magnetic recordingmedium.

Also, instead of the above-described sequential multilayer coatingmethod, it is also possible to adopt a simultaneous multilayer coatingmethod in which the magnetic layer formation coating material is appliedto the undercoat layer formation coating material and dried after theundercoat layer formation coating material is applied to the one side ofthe nonmagnetic support body and before the undercoat layer formationcoating material is dried.

There is no particular limitation on the method for applying theabove-described coating materials, and gravure coating, roll coating,blade coating, extrusion coating, or the like can be used, for example.

The following methods are examples of the method for establishing therelationship of N1/0.5>60 and N2/0.5>60 in the above-described magneticlayer, and the following methods can be implemented alone, or aplurality of the following methods can be implemented in combination.

(1) There is a method in which aggregation of the magnetic powder isreduced by connecting a continuous kneading apparatus in series behind abatch kneading apparatus, and then kneading and dispersing the magneticpowder. For example, the magnetic powder is first kneaded using thebatch kneading apparatus with a solid concentration of 70 to 90 mass %,and a kneaded material is removed by adding a solvent after kneading todilute the solid concentration to 50 to 69 mass %, whereafter thekneaded material obtained by performing kneading using the continuouskneading apparatus with a solid concentration of 25 to 50 mass % isdispersed using a sand mill. With this method, after the end ofkneading, the solid concentration when the kneaded material is removedcan be kept high, and thus aggregation of the magnetic powder can besuppressed in a dilution process in which a solvent is added afterkneading using a high shear force, and a magnetic layer having a highmagnetic powder density can be obtained.

(2) In the method in which the kneaded material is dispersed using asand mill after being kneaded using the batch kneading apparatus, thereis a method in which aggregation of the magnetic powder is reduced byperforming a pressure preliminary dispersion treatment process beforekneading. For example, a preparation treatment process is performedwhich includes a liquid mixture preparation process for preparing aliquid mixture that contains the magnetic powder, a binder, and anorganic solvent and has a solid concentration of 15 mass % or less, apressure preliminary dispersion treatment process for spraying theobtained liquid mixture from a nozzle in a pressed state using ahigh-pressure spray collision dispersion device, a concentration processfor concentrating the obtained preliminary dispersion liquid, a kneadingprocess for kneading the obtained concentrate and the binder in a statein which the solid concentration is 80 mass % or more, a dilutionprocess for diluting the obtained kneaded material using a dilutioncomponent, and a dispersion treatment process for dispersing theobtained pre-dispersion slurry having a solid concentration of 10 to 50mass % using a dispersion medium.

It is preferable that in the above-described kneading process, thepreliminary dispersion liquid is concentrated such that the solidconcentration is 80 mass % or more, and the share at the time ofkneading is 70 N·m or more.

Also, examples of the high-pressure spray collision dispersion deviceused in the pressure preliminary dispersion treatment process include adispersion device having a chamber that discharges the above-describedliquid mixture from a small nozzle by applying pressure to the liquidmixture using a high-pressure flange pump, and a dispersion devicehaving a chamber that sprays the liquid mixture from a plurality ofopposed nozzles so as to cause the mixture to undergo face-to-facecollision. Specific examples include “ALTIMIZER” (product name)manufactured by Sugino Machine Limited, a homogenizer, and a nanomizer.The pressure applied to spray the liquid mixture is preferably 50 MPa ormore and more preferably 100 MPa or more. Treatments are preferablyperformed two times or more with consideration given to a viscositydifference before and after dispersion, the particle size distributionof objects that are dispersed, prevention of a short pass of the liquidmixture, and the like. A dispersion device provided with a disk(including perforations, incisions, grooves, and the like), a pin, and aring on a stirring shaft, and a rotor rotary dispersion device (forexample, nano mill, pico mill, sand mill, dyno mill, and the like), andthe like can be used as a medium dispersion device used in thedispersion treatment process. Although the dispersion time depends onthe components of the magnetic coating material and the applications, itis preferably 30 to 90 minutes in terms of the retention time.

(3) There is a method in which aggregation of the magnetic powder isreduced by applying a high shear force to the magnetic powder using acontinuous kneading apparatus, kneading the magnetic powder, and thenkneading the magnetic powder with an appropriate shear force for a longperiod of time using a batch kneading apparatus. For example, a magneticlayer can be manufactured through a kneading process for kneadingmagnetic powder and a binder resin with a first solid concentrationusing a continuous kneading apparatus, and pulverizing the magneticpowder as much as possible by applying a high shear force to themagnetic powder so as to obtain magnetic kneaded material, and are-kneading process for kneading the kneaded magnetic material with asecond solid concentration that is less than or equal to the first solidconcentration using a batch kneading apparatus that is arranged inseries with the continuous kneading apparatus, applying a shear force toa surface of the pulverized magnetic powder, and covering the pulverizedmagnetic powder with a binder resin, which is the binder, by causing themagnetic powder to adsorb the binder resin as much as possible in astate in which the binder resin is extended. At this time, it ispreferable to set the first solid concentration in a range of 80 to 90mass %, the second solid concentration in a range of 65 to 90 mass %,and the kneading time in a range of 30 to 240 minutes.

Use of this method makes it possible to knead magnetic powder with ahigh shear force applied using the above-described continuous kneadingapparatus, and to knead a magnetic kneaded material well while takingtime using the above-described batch kneading apparatus, and thusmagnetic powder sufficiently adsorbs the binder resin, the degree ofdispersion of constituents such as magnetic powder for the magneticcoating material increases in the dispersion process, and a magneticlayer having a high magnetic powder density can be obtained.

(4) The dispersion time in the sand mill can be shortened by increasingthe degree of dispersion in the kneading process using theabove-described methods (1) to (3), and thus the occurrence ofcontamination caused by mixing of bead abrasion powder can be reducedand a magnetic layer having a high magnetic powder density can beobtained.

Also, using sand mill dispersion and a centrifugation process using acentrifuge after the dilution process makes it possible to removemagnetic powder having a predetermined particle size or more, and thusaggregates and undispersed substances are removed and a uniform magneticcoating material can be obtained. It is preferable to perform thecentrifugation process at an acceleration of 1000 to 20000 G.

Also, a magnetic coating material having a more stable dispersivenesscan be obtained by including the re-dispersion process in which themagnetic powder is further dispersed using a collision dispersiondevice, after the above-described dispersion process. A collisiondispersion device that can be used at a high pressure of 50 to 250 MPais preferable.

(5) There is a method in which a fine magnetic layer is obtained byslowly drying in the application/drying process. In particular, if thinmagnetic layers are applied and dried through sequential multilayercoating, the solvent dries due to the magnetic layers quickly drying,and thereby the magnetic layer tends to be coarse. In view of this, byincreasing the solvent concentration of air in a dry area so as to slowthe drying speed and slowly drying the magnetic layers, a magnetic layerhaving a high magnetic powder density can be obtained.

Specifically, in the magnetic layer application/drying process, it ispreferable to perform a preheating process for heating a magneticcoating film until an increase in the surface temperature of themagnetic coating film stops and the temperature reaches an approximatelyconstant, temperature, a constant-rate drying process that is performedafter the preheating process and in which the surface temperature of themagnetic coating film is kept approximately constant, and a reduced-ratedrying process that is performed after the constant-rate drying processand in which the surface temperature of the magnetic coating filmbecomes higher than the temperature at which the constant-rate dryingprocess is performed so as to solidify the magnetic coating film, and itis preferable to set the constant-rate drying period to 0.2 seconds ormore.

(6) As a method other than the above-described methods, by applying amagnetic coating material with a high solid concentration S/S, theamount of a solvent that evaporates during drying is reduced, and afiner magnetic layer can be formed.

Hereinafter, although the present invention will be described usingworking examples, the present invention is not merely limited to thefollowing working examples. Also, “parts” refers to “parts by mass” inthe description below.

Working Example 1

Preparation of Magnetic Coating Material

A mixture was prepared by mixing magnetic coating material components(1) shown in Table 1 at a high speed using a high-speed stirring mixer.Next, after the obtained mixture was subjected to dispersion treatmentusing a sand mill for 250 minutes, magnetic coating material components(2) shown in Table 2 were added so as to prepare a dispersion liquid.Next, the obtained dispersion liquid and magnetic coating materialcomponents (3) shown in Table 3 were stirred using a disperser, andfiltered using a filter so as to prepare a magnetic coating material.The solid concentration S/S of the above-described magnetic coatingmaterial was 23 mass %.

TABLE 1 Magnetic coating material components (1) parts ε-Fe₂O₃ magneticpowder, average particle diameter: 14 nm 100 Vinyl chloride-basedcopolymer (containing SO₃K group), 13.5 “MR104” manufactured by ZeonCorporation Polyurethane resin (containing SO₃Na group, glass transition8 temperature: 70° C.) “UR8200” manufactured by TOYOBO CO., LTD. Carbonblack (average particle diameter: 75 nm) 2 Particulate alumina powder(average particle diameter: 8 80 nm) Cyclohexanone 120 Toluene 120

TABLE 2 Magnetic coating material components (2) parts n-butyl stearate1 Cyclohexanone 65 Methyl ethyl ketone 65 Toluene 65

TABLE 3 Magnetic coating material components (3) parts Polyisocyanate3.5 Cyclohexanone 7 Toluene 7

Preparation of Undercoating Material

A kneaded material was prepared by kneading undercoating materialcomponents (1) shown in Table 4 using a batch kneader. Next, theobtained kneaded material and undercoating material components (2) shownin Table 5 were stirred using a disperser so as to prepare a liquidmixture. Next, a dispersion liquid was prepared by dispersing theobtained liquid mixture using a sand mill for 100 minutes, and then thedispersion liquid and undercoating material components (3) shown inTable 6 were stirred using a disperser and filtered using a filter so asto prepare an undercoating material.

TABLE 4 Undercoating material components (1) parts Needle-shaped ironoxide (average longer axis length: 110 nm) 79 Carbon black (averageparticle diameter: 17 nm) 18 Particulate alumina powder (averageparticle diameter: 3 140 nm) Vinyl chloride-hydroxypropyl acrylatecopolymer (containing 9 SO₃Na group) Polyurethane resin (containingSO₃Na group, glass transition 7.5 temperature: 20° C.) “UR8300”manufactured by TOYOBO CO., LTD. Cyclohexanone 120 Methyl ethyl ketone60 Toluene 60

TABLE 5 Undercoating material components (2) parts Stearic acid 1n-butyl stearate 1.5 Cyclohexanone 120 Toluene 120

TABLE 6 Undercoating material components (3) parts Polyisocyanate 4.5Cyclohexanone 7 Toluene 7

Preparation of Coating Material for Back Coat Layer

A liquid mixture obtained by mixing coating material components for aback coat layer shown in Table 7 was dispersed using a sand mill for 50minutes so as to prepare a dispersion liquid. 15 parts of polyisocyanatewere added to the obtained dispersion liquid, stirred, and filteredusing a filter so as to prepare a coating material for a back coatlayer.

TABLE 7 Coating material components for back coat layer parts Carbonblack (average particle diameter: 25 nm) 80 Carbon black (averageparticle diameter: 300 nm) 10 α-hematite powder (average particlediameter: 100 nm) 10 Nitrocellulose 45 Polyurethane resin (containingSO₃Na group, glass transition 30 temperature: 20° C.) “UR8300”manufactured by TOYOBO CO., LTD. Cyclohexanone 300 Methyl ethyl ketone500 Toluene 500

Preparation of Magnetic Tape for Evaluation

An undercoat layer was formed by applying the above-describedundercoating material onto a nonmagnetic support body (polyethylenenaphthalate film, thickness: 5 μm) such that the thickness of theundercoat layer after calendering was 1.1 μm, and drying theundercoating material at 100° C. Next, a magnetic layer was formed byapplying the above-described magnetic coating material onto theabove-described undercoat layer using a die coater with a coater tensionof 4.5 N/inch such that the thickness of a magnetic layer aftercalendering was 55 nm, and drying the magnetic coating material at 100°C. Thereafter, vertical alignment treatment was performed while anoriented magnetic field (450 kA/m) was applied using a solenoid magnet.

Next, a back coat layer was formed by applying the above-describedcoating material for a back coat layer to the undercoat layer of thenonmagnetic support body and a surface that was opposite to the surfaceprovided with the magnetic layer such that the thickness aftercalendering was 0.5 μm, and drying the coating material at 100° C.

Thereafter, the undercoat layer and the magnetic layer were formed onthe upper surface of the nonmagnetic support body, a whole cloth rollprovided with the back coat layer on its lower surface was subjected tocalendering using a calender apparatus having seven stages of metalrolls at a temperature of 100° C. and a linear pressure of 300 kg/cm.

Lastly, a magnetic sheet was produced by subjecting the obtained wholecloth roll to hardening treatment at 60° C. for 48 hours. A magnetictape for evaluation was produced by cutting the obtained magnetic sheetinto ½-inch width.

Working Example 2

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material that was produced in WorkingExample 1 was applied such that the thickness of a magnetic layer aftercalendering was 135 nm.

Working Example 3

The ε-Fe₂O₃ magnetic powder in the magnetic coating material components(1) shown in Table 1 was changed to one having an average particlediameter of 12 nm, the changed magnetic coating material components (1)were mixed at a high speed using a high-speed stirring mixer to preparea mixture. Next, after the obtained mixture was subjected to dispersiontreatment using a sand mill for 250 minutes, re-dispersion treatment wasfurther performed using a collision dispersion device. Thereafter, themagnetic coating material components (2) shown in Table 2 were addedsimilarly to Working Example 1 so as to prepare a dispersion liquid.Next, the obtained dispersion liquid and magnetic coating materialcomponents (3) shown in Table 3 were stirred using a disperser, andfiltered using a filter so as to prepare a magnetic coating material.

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material was applied such that thethickness of a magnetic layer after calendering was 85 nm.

Working Example 4

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material that was produced in WorkingExample 1 was applied such that the thickness of a magnetic layer aftercalendering was 85 nm and dried at 100° C. to form a magnetic layer, andthen the vertical alignment treatment was performed while an orientedmagnetic field (900 kA/m) was applied using a solenoid magnet.

Working Example 5

A magnetic coating material was prepared similarly to Working Example 1except that the ε-Fe₂O₃ magnetic powder in the magnetic coating materialcomponents (1) shown in Table 1 was changed to one having an averageparticle diameter of 16 nm. A magnetic tape for evaluation was producedsimilarly to Working Example 1 except that the magnetic coating materialwas applied such that the thickness of a magnetic layer aftercalendering was 85 nm.

Working Example 6

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material that was produced in WorkingExample 1 was applied such that the thickness of a magnetic layer aftercalendering was 85 nm and dried at 100° C. to form a magnetic layer, andthen the vertical alignment treatment was performed while an orientedmagnetic field (300 kA/m) was applied using a solenoid magnet.

Working Example 7

A magnetic tape for evaluation was produced similarly to Working Example1 except that an undercoat layer and a magnetic layer were formed on theupper surface of a nonmagnetic support body by applying the magneticcoating material that was produced in Working Example 1 such that thethickness of a magnetic layer after calendering was 85 nm, a whole clothroll provided with a back coat layer on its lower surface was subjectedto calendering using a calender apparatus having seven stages of metalrolls at a temperature of 90° C. and a linear pressure of 300 kg/cm.

Comparative Example 1

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material that was produced in WorkingExample 1 was applied such that the thickness of a magnetic layer aftercalendering was 43 nm.

Comparative Example 2

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material that was produced in WorkingExample 1 was applied such that the thickness of a magnetic layer aftercalendering was 150 nm.

Comparative Example 3

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material that was produced in WorkingExample 1 was applied such that the thickness of a magnetic layer aftercalendering was 85 nm and dried at 100° C. to form a magnetic layer, andthen the vertical alignment treatment was performed while an orientedmagnetic field (100 kA/m) was applied using a solenoid magnet

Comparative Example 4

A magnetic coating material was prepared similarly to Working Example 1except that the ε-Fe₂O₃ magnetic powder in the magnetic coating materialcomponents (1) shown in Table 1 was changed to one having an averageparticle diameter of 18 nm. A magnetic tape for evaluation was producedsimilarly to Working Example 1 except that the magnetic coating materialwas applied such that the thickness of a magnetic layer aftercalendering was 85 nm.

Comparative Example 5

A magnetic tape for evaluation was produced similarly to Working Example1 except that the magnetic coating material was applied with the amountsof cyclohexanone, methyl ethyl ketone, and toluene, which were thesolvent components in the magnetic coating material components (2) shownin Table 2, being changed to 100 parts each, the solid concentration S/Sof the magnetic coating material being 20 mass %, and the die coaterhaving a coater tension of 2.5 N/inch.

Comparative Example 6

A magnetic coating material was prepared similarly to Working Example 1except that the ε-Fe₂O₃ magnetic powder in the magnetic coating materialcomponents (1) shown in Table 1 was changed to one having an averageparticle diameter of 12 nm, the amounts of cyclohexanone, methyl ethylketone, and toluene, which were the solvent components in the magneticcoating material components (2) shown in Table 2, were changed to 100parts each, and the solid concentration S/S of the magnetic coatingmaterial was 20 mass %. A magnetic tape for evaluation was producedsimilarly to Working Example 1 except that the magnetic coating materialwas applied with the die coater having a coater tension of 2.5 N/in suchthat the thickness of a magnetic layer after calendering was 85 nm. Notethat in Comparative Example 6, the average particle diameter of theε-Fe₂O₃ magnetic powder was reduced to 12 nm similarly to WorkingExample 3, but unlike Working Example 3, re-dispersion treatment using acollision dispersion device was not performed.

Next, the following properties were measured using the produced magnetictape for evaluation.

Mr·t in thickness direction of magnetic layer, magnetic coercive force,and squareness ratio

A hysteresis curve of the magnetic tape for evaluation was obtainedusing a vibrating sample magnetometer “VSM-P7” (product name)manufactured by TOEI INDUSTRY CO., LTD. Mr·t in the thickness directionof the magnetic layer, the magnetic coercive force, and the squarenessratio were obtained based on the above-described hysteresis curve.Specifically, the magnetic tape for evaluation was cut into a circlehaving a diameter of 8 mm to prepare a cut sample, 20 cut samples werestacked with the thickness direction of the magnetic tape coincidingwith the direction in which the external magnetic field was applied, soas to prepare a measurement sample. As a data plot mode of the vibratingsample magnetometer, the applied magnetic field was set to −16 kOe to 16kOe, a time constant TC was set to 0.03 sec, the drawing step was set to6 bits, and the wait time was set to 0.3 sec.

N1/0.5 and N2/0.5

As shown in FIG. 2, the straight line W and the straight line L weredisplayed on a photograph obtained by imaging the surface of themagnetic layer using a scanning electron microscope (SEM) “S-4800”manufactured by HITACHI Ltd. with an acceleration voltage of 2 kV,10000-fold (10 k-fold) magnification, and an observation condition ofU-LA100, N1 and N2 were obtained using the above-described method, andN1/0.5 and N2/0.5 were obtained from those values.

Specifically, images at 10 k-fold magnification were captured in 5fields of view (five images in total) at different locations on thesurface of the magnetic layer, one straight line W and one straight lineL were displayed on each image, a value obtained by dividing an N1average value of the five straight lines W by 0.5 μm was deemed to beN1/0.5, and a value obtained by dividing an N2 average value of the fivestraight lines L by 0.5 μm was deemed to be N2/0.5.

Spacing of Magnetic Layer

Spacing was measured using a TSA (tape spacing analyzer) manufactured byMicro Physics after the surface of the magnetic layer was cleaned usingn-hexane.

Specifically, the pressure at which the magnetic layer was pressedagainst a glass plate using a urethane hemisphere was 0.5 atm (5.05×10⁴N/m). A certain region (240000 to 280000 μm²) of the surface of themagnetic tape for evaluation on the magnetic layer side was irradiatedthrough the glass plate with white light from a stroboscope in thatstate, the light reflected therefrom passed through an IF filter (633nm) and an IF filter (546 nm) and was received by a CCD, and thereby aninterference fringe image caused by the unevenness of that region wasobtained.

Next, that image was divided into 66000 points and distances from theglass plate at each point to the surface of the magnetic layer wereobtained and used as a histogram (frequency curve), the histogram wasformed into a smooth curve through lowpass filter (LPF) processing, andthe distance from the glass plate at the peak position to the surface ofthe magnetic layer was deemed to be the spacing.

Also, optical constants (phase, reflectance) of the surface of themagnetic layer that was used to calculate the above-described spacingwere measured using a reflective spectral film thickness meter “FE-3000”manufactured by OTSUKA ELECTRONICS, CO., LTD., and a value near awavelength of 546 nm was used.

The magnetic tape for evaluation was cleaned using n-hexane throughultrasonic cleaning at room temperature for 30 minutes by immersing themagnetic tape for evaluation in n-hexane.

Output Properties

An inducible/GMR complex magnetic head having a writing track width of 5μm and a readout track width of 2.3 μm was attached to a linear tapeelectromagnetic conversion property measurement apparatus that wasproduced by modifying an LTO drive, and evaluation was performed byrecording signals having a recording wavelength of 200 nm (G7×1.05-foldlinear recording density) in the magnetic tape at a tape speed of 1.5m/sec.

The apparatus had a traveling system in which magnetic heads areattached at two locations, and thus two magnetic heads described abovewere attached. The magnetic heads were placed on a precise piezo stage(having a movement resolution of 10 nm) that was movable in the trackwidth direction, the upstream magnetic head recorded signals and thedownstream magnetic head eliminated an alternating current in oneinstance of traveling, and signals having a magnetization width of 0.8μm were produced on the magnetic tape by offsetting the upstreammagnetic head and the downstream magnetic head in the track widthdirection by 0.8 μm.

Next, after signals were reproduced by causing the magnetic tape totravel again, and the reproduced signals were amplified using acommercially available read amplifier for an MR head, a basic wavecomponent output (S) of the signals and integral noise (N) from afrequency of the basic wave component output to a frequency of two timesthe basic wave component output were measured using a spectrum analyzer“N9020A” manufactured by Keysight Technologies (formerly, AgilentTechnologies, Inc.). The above results are indicated using relativevalues (dB) using the S/N ratio of Comparative Example 1 as the standard(0 dB).

Length of Magnetization in Width Direction of Magnetic Layer

The length of magnetization in the width direction of a signal recordedin the magnetic layer when the above-described output properties weremeasured was measured as follows. That is, the length of themagnetization in the width direction of the magnetic layer on whichsignals are recorded was measured using a frequency detection method and“NANO SCOPE III” manufactured by Digital Instruments Corporation as amagnetic force microscope. A probe having cobalt alloy coating (theradius of curvature of the tip: 25 to 40 nm, magnetic coercive force:about 400 [Oe], magnetic moment: about 1×10⁻¹³ emu) was used as themeasurement probe, the scanning range was 5 μm×5 μm, and the scanningspeed was 5 μm/sec.

The evaluation results above are shown in Tables 8 and 9.

TABLE 8 Average particle Length of Squareness diameter of magnetizationratio in magnetic in width Mr · t thickness particles direction (μm) (μT· m) direction N1/0.5 N2/0.5 (nm) Working 0.8 0.0013 0.70 70 75 14Example 1 Working 0.8 0.0032 0.70 71 82 14 Example 2 Working 0.8 0.00200.70 83 90 12 Example 3 Working 0.8 0.0020 0.75 67 76 14 Example 4Working 0.8 0.0020 0.70 60 60 16 Example 5 Working 0.8 0.0020 0.66 68 7814 Example 6 Working 0.8 0.0020 0.70 73 81 14 Example 7 Comparative 0.80.0010 0.70 72 79 14 Example 1 Comparative 0.8 0.0034 0.65 66 60 14Example 2 Comparative 0.8 0.0020 0.63 70 80 14 Example 3 Comparative 0.80.0020 0.70 56 58 18 Example 4 Comparative 0.8 0.0020 0.70 59 57 14Example 5 Comparative 0.8 0.0020 0.70 58 59 12 Example 6

TABLE 9 Magnetic coercive force in thickness direction TSA spacing (Oe)(nm) SN ratio Working 3200 10 0.9 Example 1 Working 3200 10 0.5 Example2 Working 3200 10 2.9 Example 3 Working 3200 10 1.6 Example 4 Working3200 10 1.0 Example 5 Working 2800 10 1.2 Example 6 Working 3200 14 0.6Example 7 Comparative 3200 10 0.0 Example 1 Comparative 3200 10 0.1Example 2 Comparative 3200 10 0.3 Example 3 Comparative 3200 10 −0.5Example 4 Comparative 3200 10 −0.3 Example 5 Comparative 3200 12 −0.2Example 6

Based on Tables 8 and 9, it was found that the SN ratios of WorkingExamples 1 to 7 according to the present invention were higher thanthose of Comparative Examples 1 to 6.

In Working Example 1, the magnetic coating material was applied with theamount of cyclohexanone, methyl ethyl ketone, and toluene that were thesolvent components in the magnetic coating material components (2)reduced from the conventional 100 parts to 65 parts in order to increasethe density of magnetic particles, and the solid concentration S/S atthe time of application increased from the conventional 20 mass % to 23mass %. By increasing the solid concentration S/S, the viscosity of themagnetic coating material increased and it became difficult to apply athin magnetic layer, but attempts were made to increase the precision ofan applicator, the coater tension was increased from the conventional2.5 N/inch to 4.5/inch, and the magnetic coating material was applied,and thereby a uniform thin magnetic layer was obtained.

By applying the magnetic coating material with a high solidconcentration S/S in this manner, the amount of solvents evaporatingduring drying was reduced, and a fine magnetic layer was obtained withminute particle powder, such as ε-Fe₂O₃ magnetic particles.

In Working Example 2, the value of Mr·t increased due to an increase inthe thickness of the magnetic layer to about 2.5 times the thickness ofWorking Example 1, and the share was smoothly applied from the die lipto the surface layer of the magnetic layer in the longitudinal directiondue to a thick magnetic layer, and thus the magnetic layer became finein the longitudinal direction and N2/0.5 increased.

In Working Example 3, the value of Mr·t increased because the thicknessof the magnetic layer was about 1.5 times thicker than in WorkingExample 1, and re-dispersion treatment was performed on the magneticcoating material using the collision dispersion device, and thus thedispersiveness of magnetic particles increased and N1/0.5 and N2/0.5increased.

In Working Example 4, the value of Mr·t increased because the thicknessof the magnetic layer was about 1.5 times thicker than in WorkingExample 1, and the strength of the oriented magnetic field was increasedcompared to Working Example 1, and thus the squareness ratio increased.

In Working Example 5, the value of Mr·t increased because the thicknessof the magnetic layer was about 1.5 times thicker than in WorkingExample 1, and N1/0.5 and N2/0.5 decreased because ε-Fe₂O₃ magneticpowder having a larger average particle diameter than in Working Example1 was used.

In Working Example 6, the value of Mr·t increased because the thicknessof the magnetic layer was about 1.5 times thicker than in WorkingExample 1, and the strength of the oriented magnetic field was reducedcompared to Working Example 1, and thus the squareness ratio and themagnetic coercive force decreased.

In Working Example 7, the value of Mr·t increased because the thicknessof the magnetic layer was about 1.5 times thicker than in WorkingExample 1, and the TSA spacing increased because the calenderingtemperature decreased compared to Working Example 1.

In Comparative Example 1, the value of Mr·t decreased because thethickness of the magnetic layer was reduced compared to Working Example1.

In Comparative Example 2, the value of Mr·t increased because thethickness of the magnetic layer was about 2.7 times thicker than inWorking Example 1 and exceeded 0.0032 μT·m.

In Comparative Example 3, the value of Mr·t increased because thethickness of the magnetic layer was about 1.5 times thicker than inWorking Example 1, and the squareness ratio decreased because thestrength of the oriented magnetic field decreased compared to WorkingExample 1.

In Comparative Example 4, the value of Mr·t increased because thethickness of the magnetic layer was about 1.5 times thicker than inWorking Example 1, and N1/0.5 and N2/0.5 decreased and were lower than60 because ε-Fe₂O₃ magnetic powder having a larger average particlediameter than in Working Example 1 was used.

In Comparative Example 5, the amount of solvent evaporating in thedrying process increased because the solid concentration S/S of themagnetic coating material decreased compared to Working Example 1, andas a result, the magnetic layer became slightly coarse, N1/0.5 andN2/0.5 decreased and were lower than 60.

In Comparative Example 6, ε-Fe₂O₃ magnetic powder having a smalleraverage particle diameter was used similarly to Working Example 3, butthe minute magnetic particle powder was not sufficiently dispersedbecause the re-dispersion treatment using the collision dispersiondevice was not performed, and the amount of the solvents evaporating inthe drying process increased because the solid concentration S/S of themagnetic coating material decreased compared to Working Example 1, andas a result, the magnetic layer became slightly coarse, N1/0.5 andN2/0.5 decreased and were lower than 60.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which fall within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A magnetic recording medium comprising: a nonmagnetic support body; and a magnetic layer containing magnetic particles, wherein 0.0013 μT·m<Mr·t<0.0032 μT·m is satisfied where Mr is a residual magnetic flux density in a thickness direction of the magnetic layer and t is an average thickness of the magnetic layer, a squareness ratio in the thickness direction of the magnetic layer is 0.65 or more, if a straight line W having a length of 500 nm and a width of 15 nm is displayed parallel to a width direction of the magnetic layer and a straight line L having a length of 500 nm and a width of 15 nm is displayed parallel to a longitudinal direction of the magnetic layer, on an image obtained by observing a surface of the magnetic layer using a scanning electron microscope at 10 k-fold magnification, the straight line W does not intersect a particle having a particle diameter of 50 nm or more and a gap having a maximum width of 50 nm or more on the image, and on the image, the number of magnetic particles that intersect the straight line W is N1, and the straight line L does not intersect a particle having a particle diameter of 50 nm or more and a gap having a maximum width of 50 nm or more on the image, and on the image, the number of magnetic particles that intersect the straight line L is N2, then, a relationship of N1/0.5>60 and N2/0.5>60 is established where N1/0.5 is the number of magnetic particles per micrometer obtained by dividing N1 by 0.5 μm and N2/0.5 is the number of magnetic particles per micrometer obtained by dividing N2 by 0.5 μm.
 2. The magnetic recording medium according to claim 1, wherein if a length of magnetization in the width direction of the magnetic layer is set to 1 μm or less, the length being a length of magnetization of a signal recorded in the magnetic layer, reproduction is performed by a TMR head.
 3. The magnetic recording medium according to claim 1, wherein the magnetic particles are made of ε-iron oxide.
 4. The magnetic recording medium according to claim 3, wherein an average particle diameter of the magnetic particles made of the ε-iron oxide is 15 nm or less.
 5. The magnetic recording medium according to claim 1, wherein a magnetic coercive force in the thickness direction of the magnetic layer is 3000 oersteds [Oe] or more.
 6. The magnetic recording medium according to claim 1, wherein when spacing on a surface of the magnetic layer is measured using a TSA (tape spacing analyzer) after the surface of the magnetic layer is cleaned using n-hexane, a value of the spacing is 5 nm or more and 12 nm or less.
 7. The magnetic recording medium according to claim 1, wherein a thickness of the magnetic layer is 30 nm or more and 200 nm or less. 